On the oxidation of stainless steel particles in the plasma jet

Abstract

The in-flight oxidation of stainless steel particles in the plasma jet was investigated in this work. Two types of 316L austenitic stainless steel particles were sprayed by a dc plasma gun in ambient or controlled atmosphere varying gun parameters and surrounding gases composition. The in-flight collected particles were characterized to establish relationship between spray parameters and particle oxidation behavior. The in-flight particle oxidation mechanisms were then suggested. It was shown that besides diffusion based oxidation, convective oxidation in the particles can occur within the plasma jet core if plasma to particle kinematic viscosities ratio and relative Reynolds number (Re) are superior to 55 and 20, respectively. In these conditions, the oxide formed or oxygen dissolved at the surface of the liquid particle can be swept into its interior forming isolated oxide nodules. Fresh liquid metal is transported from interior towards particle surface. The oxidation rates were estimated to be higher compared to diffusion based oxidation which was found to be the dominant phenomenon in the plasma jet plume in the absence of convective oxidation. Spray parameters leading to higher kinematic viscosities ratio and Re, such as increasing arc current, hydrogen content in the plasma forming gases, or decreasing sprayed particle size range, resulted in enhanced convective oxidation in the plasma core. The diffusion based oxidation of particles in the plasma jet plume can be principally controlled by their size (specific surface area), temperature and velocity (dwell time) and the molar fraction of oxidizing and reducing species in the plasma jet. While investigating the influence of the atmosphere of plasma jet on the in-flight particle oxidation, it was found that the surface area of the oxide nodules and the mass percentage of total oxygen in collected particles followed a parabolic and linear relationship with p_O₂ in the surrounding atmosphere. Keeping surrounding p_O₂ at 0.1 and altering N₂ and Ar content resulted in higher oxygen content in particles sprayed in Ar rich surrounding whereas no distinct difference in oxide nodules surface area was measured.